Electrical power generation employs turbine systems including components such as stationary and rotating blades, pipes and impellers, which are subject to the harsh erosive effects of hard particles present in steam and other high temperature gases. The in-service performance of these components is often governed by the mechanical properties of the substrate alloy, such as fatigue resistance, creep resistance, and tensile strength.
Preferred alloys used in manufacturing turbine components include high strength steels, low alloy steels, and stainless steels. Although exhibiting high strength, these materials are relatively prone to erosion and can be severely eroded when exposed to pressurized, high-temperature steam containing solid particles for extended periods of time. This erosion has been known to increase in severity until the component is no longer useful and must be replaced.
Accordingly, since the down-time associated with replacing components in a power turbine can run into hundreds of thousands of dollars, there is a pressing need for reducing erosion of steam turbine components.
One art-recognized procedure for minimizing the effects of erosion is to provide a relatively hard erosion-resistant surface, such as a boride coating, to the turbine component. See Hayes, U.S. Pat. No. 3,935,034 which is hereby incorporated by reference. Hayes discloses components treated with a pack cementation boride diffusion process in which the component is placed in a sealed box containing boron and an inert filler. The contents of the sealed box are then heated to a temperature of greater than about 1350.degree. F. After being subjected to this temperature for a period of hours, the contents of the sealed box are cooled to room temperature. During this elevated temperature, the boron diffuses into the substrate steel of the component forming an boride coating consisting of iron boride and chromium boride intermetallic compounds on the steel substrate.
Studies have demonstrated that the erosion resistance of coated stainless steel test specimens including boride or carbide coatings is very dependent on the process parameters employed in depositing the coatings. See Qureshi, et al., "Characterization of Coating Processes and Coatings for Steam Turbine Blades", Journal of Vacuum Science and Technology, 2nd Series, Vol. 4, No. 6, p.p. 2638-2647 (Nov./Dec. 1986); Qureshi and Tabakoff, "The Influence of Coating Processes and Process Parameters on Surface Erosion Resistance and Substrate Fatigue Strength", Surface and Coatings Technology, 36, pp. 433-444 (1988).
While boride coatings provide an erosion resistant surface to steam turbine components, they often develop surface cracks and imperfections during required post coating heat treatment operations. These imperfections can seriously impair the fatigue strength of the steel substrate as well as the erosion life of the boride coating, thus minimizing the coating's potential as a protective surface. Moreover, boride coatings are known to oxidize at elevated service temperatures, resulting in spalling of the coating from the turbine component surfaces.